"Easter Egg" is the name regularly used by programmers to describe something hidden in plain sight within a software program. As this cache was hidden on Easter; and is indeed an egg; we thought it would be the best way do describe it as the Easter Egg - as it also shows a unique place right up at the centre of Sofia.
A train station.
You heard that well. The old "Serdika" train station was here; and the wall where the cache is; is the original one, covering the tracks from falling stones and mud form the nearby hill. Walk around - you may also find where the tracks have been (it is visible). And the paved road you walked on to reach the cache was once where trucks arrived to loaded/unloaded at the station docks.
An easter egg in Sofia!